The invention relates to a device for introducing digital data into a medium by way of a status quantity thereof having at least two values, said device comprising a clock input for receiving a clock pulse series, a data input for receiving, in synchronism with the clock pulse series, a flow of data bits in a succession of bit cells, a sequentially operating logic circuit having means for being switched between at least three positions thereof, under the control of a data bit received, in order to generate per position a code bit, the first value of a data bit (1) alternatively corresponding to a first (+1) and a second (-1) value, respectively, of a code bit, the second value of a data bit (0) corresponding to the third value of a code bit (0), and to apply these code bits to the medium, by way of a code output device, in a self-synchronising series, at least substantially without d.c. component. The medium may be a magnetizable material which is driven along a read/write head device. Introduction thereby implies storage. Also, the medium may be a channel for data transmission. Introduction thereby means transmission. A code system of the described kind is known from a paper by P. J. van Gerwen, "On The Generation And Application Of Pseudo-Ternary Codes In Pulse Transmission ", Phil. Res. Repts. 20 (1965), 469-484, notably FIGS. 7, 8, 9. Such a code without d.c. component is attractive when the transmission path to or the medium itself does not accept a d.c. component. This is the case, for example, for a transformer. A further advantage of said property consists in that a base line shift cannot have adverse effects on the accuracy during decoding. When a suitable digital filter is used, the known code has further zero points in the frequency response curve. The surroundings of one such zero point may be used for inserting a pilot frequency in the signal. A further advantage of the known device may consist in that no feed-forward is present, but feedback, so that an error will not be propagated in the flow of code bits. This property would decrease the sensitivity against interference or simplify the correction. The described code, uses three amplitude levels. In magnetic recording, this is a drawback because the intermediate level can never have magnetic saturation, so that the code is not self-erasing: an additional erase operation is thus required before introduction of new data. Moreover, the susceptibility to interference is reduced by the use of three different amplitude levels.